Abstract

The most striking sea surface temperature (SST) phenomenon in the tropical Atlantic is the seasonal appearance of the Atlantic Cold Tongue (ACT). The ACT, characterized by strongly reduced temperatures, develops in late boreal spring/early summer with the strengthening of the southeast trades along the equator and last until late fall/early winter. Onset, duration, spatial extent and strength of cooling vary significantly from year to year. The mixed layer (ML) heat budget terms in the ACT region are rather well estimated, but only locally or with model results. The role of near-surface salinity variations for ACT onset and development as well as the mixed layer salinity budget are less understood.In this thesis ML temperature and salinity (MLS) changes during ACT development were studied from May to July 2011 by a cold tongue experiment (CTE). The CTE was based on two successive research cruises, a glider swarm experiment, and moored observations. This in-situ dataset together with satellite data, atmospheric reanalysis data, and assimilation model output were used to evaluate the ML heat and salinity budget for two sub-regions: 1) the western ACT between 23°-10°W showing strong cooling during the CTE and 2) the region north of the ACT influenced by the northward migrating Intertropical Convergence Zone. The findings of the year 2011 were compared with mean seasonal cycles at three buoy sites (0°N, 23°W; 4°N, 23°W; 0°N, 10°W).The strong ML heat loss in the ACT region during the CTE was found to be the result of the balance between warming due to net surface heat flux and cooling due to diapycnal mixing and zonal advection. The dominant role of diapycnal mixing was observed as well as conjectured from the residual of the heat budget by using estimated diapycnal diffusivities and vertical temperature gradients at the base of the ML. In the region north of the ACT the weak cooling was achieved by similar contributions of net surface heat flux, zonal advection and entrainment. Only a small residual remained in the region north of the ACT.During the CTE, salinity in the ACT region slightly increased with a balance of overall freshening due to strongly varying zonal advection and salinity increase due to the net surface freshwater flux. A strong salinity increase in the ACT region occurred at the equator, 10°W before the CTE and was not captured by our measurements. The diapycnal mixing, which act to erode the high salinity core of the Equatorial Undercurrent, could only partly be related to the remaining residual. In the region north of the ACT, stronger precipitation resulted in a freshening effect due to a net surface freshwater flux. Zonal advection changed sign during the CTE contributing to a ML freshening at the beginning of the CTE and a salinity increase afterward.The salinity balance at the equator is characterized by weak seasonal changes. In the first half of the year the MLS changes are caused by the freshening due to precipitation, zonal advection and eddy salt advection, which are balanced by the salinity increasing contribution of evaporation, meridional salt advection and entrainment. In the western cold tongue region, a diapycnal salt flux during February/March and in June is a salt gain for the ML. Meridional salt advection increases MLS during the second half of the year in the cold tongue region. In particular in the central cold tongue region an imbalance during summer and fall remains. In summer this imbalance can be minimized by the implementation of the diapycnal salt flux. At the northern buoy at 4°N the seasonal cycle of MLS is caused by the semiannual cycle of precipitation and meridional advection and the annual cycle of entrainment and eddy salt advection.